Carbon nanotubes (CNTs) coated by diamond nano-crystals (NCD) in a conformal manner are hybrid nanostructured carbon materials. They combine the structural, physical and chemical properties of both carbon allotropes in a wide range of applications. CNTs exhibit high tensile strength, radial elastic deformability and toughness, together with unique electronic transport properties. NCD films display high hardness and stiffness, low coefficient of friction, exceptional chemical inertness, biocompatibility, high thermal conductivity, negative electron affinity with proper treatment and high transparency in a wide range of wavelengths. CNTs coated with NCD in a conformal manner can provide advantageous properties such as thermo-resistance and enhanced field emission current properties including stable emission currents. They can be applied in diverse fields such as functional composite materials, field emissions and other electronic devices, biodevices, wear-resistant coatings, thermal management of integrated circuits, electrical field shielding and micro- and nano-electromechanical systems (MEMS/NEMS).
A few processes have been disclosed for elaborating nano-structured carbon material with diamond nanocrystals. However, these processes did not yield carbon nanotubes conformally coated with diamond nanocrystals (U.S. Patent Application Publication No. 2006/0222850 A1; Xiao et al., 2005, Advanced Materials 17:1496-1500; Shankar et al., 2008, Diamond and Related Materials 17:79-83).
Sun et al. obtained diamond nanocrystals (Sun et al., 2004, Applied Physics Letters 84:2901-2903) from multi-walled CNTs (MWCNTs) exposed for 10 hours to hydrogen plasma at a high substrate temperature, TS, of 727° C. At this high temperature, the resulting CNTs were partially converted into amorphous carbon and thus were no longer tubular. Diamond nano-particle diameter ranged from 5 to 30 nm and corresponding nucleation density was estimated to be 1011/cm2. If MWCNT plasma exposure duration was increased to 20 hours, diamond nano-particle grew into diamond nanowires, but diamond nucleation density was not increased significantly (Sun et al., 2004, Advanced Materials 16:1849-1853).
Shankar et al. used hot-filament chemical vapor deposition (HFCVD) to synthesize nanometer-sized diamond particles (2 to 20 nm) nucleating and growing radially outward on the surface of MWCNTs which were previously pre-dispersed onto a silicon substrate. They identified a small parametric window in the diamond growth space-phase (2-5% CH4 in H2) wherein the CNTs were not destroyed and wherein their structure was partially preserved. As shown in FIG. 1 of Shankar et al., conformal coating was not achieved. In order to enhance nucleation density of diamond on CNTs, Shankar et al. studied the effects of substrate temperature, precursor concentration, CNT film thickness and pressure on nucleation density of diamond on CNTs. The conditions for maximum diamond nucleation density were found to correspond to cases where the CNTs were almost completely etched away by hydrogen radicals, which would indicate a limitation to achieve higher diamond nucleation density by this method and therefore a limitation to achieve conformal coating (Nagraj Shankar, 2004, PhD dissertation, University of Illinois at Urbana-Champaign). Indeed, CNTs have been used to enhance diamond nucleation on various substrates, in particular on substrates which neither dissolve carbon nor form carbide, such as copper; however, CNTs have not been shown to survive such processes (Hou et al., 2002, Applied Surface Science 185:303-308).
Terranova et al. reported on the growth of single-walled CNT (SWCNT) bundles coated by diamond nano-crystals in a conformal manner, at a high substrate temperature (TS=900° C.), in one step, by a modified HFCVD process (Terranova et al., 2005, Chemistry of Materials 17:3214-3220). Carbon nano-powders were sprayed from holes along a nozzle. The spray intersected with a flux of atomic hydrogen from a heated filament that was located 6 mm away from a Si substrate coated with catalyst Fe nano-particles for the growth of CNTs. SWCNT bundles (diameter <120 nm) were formed first, followed by NCD coating. The reported size of the diamond grains with well-defined crystalline facets was 20-100 nm. However, this method suffers from important drawbacks. The substrate temperature (TS=900° C.) was too high to maintain the integrity of the hybrid CNT/nanodiamond material with temperature sensitive substrate materials. The high synthesis substrate temperature (TS=900° C.) is prohibitive for low cost mass production. The method may suffer from lack of homogeneity and high potential contamination from the nozzle used in the setup. The setup includes additional components, as compared to conventional HFCVD process, which increase production cost. The method is not appropriate for mass production. The size of the diamond grains is 20-100 nm, which may result in insufficient conformal coating of CNT bundles. Sub 10 nm diamond grains are desirable to achieve a higher degree of conformal coating. For these reasons, the method disclosed by Terranova et al. might not be suitable to be scaled up and adapted to the requirements of various industries such as the electronics industry.
Later, Terranova et al., while exploring other routes of preparation of hybrid CNT/nanodiamond structures, which would be suitable to be scaled up and adapted to the requirements of the electronics industry, reported the coating of SWCNT bundles by a standard HFCVD process (Guglielmotti et al., 2009, Applied Physics Letters 95:222113). The SWCNT layers were deposited on Si(100) plates by drop casting of dispersions prepared adding 2 mg of the purified materials to 25 ml of methanol. The coating of the SWCNT bundles by diamond was carried out in an HFCVD reactor where the gaseous phase was activated by a tantalum (Ta) wire kept at 2200° C. The reactant used was a mixture of 1% CH4 diluted in H2. Gas flow and working pressure were 200 standard cubic centimeters per minute and 36 Torr, respectively. Substrate temperature was estimated to be 630° C. The HFCVD process was performed for 30 min. According to the authors, the SWCNT bundles appeared uniformly coated by nano-grains, typically of the order of 10 nm. However, the published low-magnification scanning electron microscopy (SEM) images did not allow estimation of mean grain size. Transmission electron microscopy images were not provided. Reflection high energy electron diffraction analysis revealed rings characteristic of diamond phase. There was no commentary on the possible deposition of nano-crystalline SiC, which may form under those conditions, onto the CNT and substrate surface. From these results, it is not clear whether the diamond was deposited on the silicon substrate surface or on the surface of the SWCNT bundles. In any case, the substrate temperature remains prohibitive for various applications for which CNTs must be integrated with temperature sensitive substrate materials.
Conformal coating of CNTs with silicon carbide (SiC) is also of great interest for improving the chemical and physical properties of CNTs. For instance, CNTs conformally coated with SiC may be used for improving the thermo-oxidative stability of CNTs used as nano-reinforcements for metal, ceramic or polymer matrixes, or for improving the electron emission stability of CNTs used in field emission devices. Nanocrystalline SiC exhibits high elasticity, strength, chemical inertness, wide band gap, high electron mobility and thermal conductivity. However, similar to attempts to conformally coat CNTs with diamond, attempts to conformally coat CNTs with SiC have been performed at very high CNT temperatures that are impractical for many applications. For example, MWCNTs (40-1000 nm outer diameter) were coated with a nanometer-sized SiC layer by the reaction of SiO(g) and CO(g) at temperatures of 1150-1550° C. in vacuum (Morisada & Miyamoto, 2004, Materials Science and Engineering A 381:57-61). More recently, a similar result was achieved with MWCNTs of 15 nm external diameter, using polycarbosilane as precursor, heated at about 1300° C. under an inert atmosphere (Gupta et al., 2009, Silicon 1:125-129). As described above, the temperature used in these processes compromises the integrity of the CNTs and is completely incompatible with temperature-sensitive substrates.
In conclusion, until now, no method has been available to provide CNTs coated by diamond nanocrystals and/or SiC in a conformal manner at low pressure (around 10 Torr) and low temperature (below 360° C.) that is suitable to be scaled up and adapted to the requirements of various industries such as the electronics industry. In the methods to date, the high temperature required for the substrate and CNTs (above 630° C.) has made it impossible to coat CNTs located on many materials that would be destroyed or affected by this temperature. This restriction includes all polymeric materials, as well as many semiconductors and device structures.
There is need for an improved method of manufacturing CNTs coated by diamond nano-crystals and/or SiC in a conformal manner that will overcome the problems noted above to reduce cost, reduce waste, reduce components, improve conformal degree, and create the opportunity to deposit on temperature sensitive materials.